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E39 (1997 - 2003) The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

The readout resets every time you turn off the ignition. So, to read parameters whilst driving, first start the engine, and then activate the readout and call up the parameters of interest. They'll stay up there all the time the engine is running.

There is a trick you can use to cross check the 2 thermistors in the dual temp sensor in the engine.

Start the engine, unlock the cluster and step through the menu to engine temperature. Turn the engine off by turning the key from the "2" position to "1" position but not the "0" position as to remove the key. The unlocked cluster test stays active in the display, but the reading at position 1 will likely be different from 2 by a degree or two. You can toggle between 1 & 2 to see them swap. Works hot engine or cold.

It there is a significant difference between the readings, it's time to change the DTS.

Apparently the display is switched between the thermistor that drives the gauge & the temp signal from the DME which reads the other thermistor in the DTS. I've no idea which is which. Nor whether the gauge needle and overtemp warning light share a common input or one is drive by the DME and the other from the thermistor.